spiffs: Document how to generate images

fs/spiffs/README.md

@@ -0,0 +1,25 @@
+# SPIFFS
+
+## Creating an image
+
+This implementation is supposed to be compatible with
+images generated by the following tools:
+
+* mkspiffs <https://github.com/igrr/mkspiffs>
+* ESP-IDF spiffsgen.py <https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/storage/spiffs.html#spiffsgen-py>
+
+Note: please ensure the following NuttX configs to be compatible with
+these tools:
+
+* CONFIG_SPIFFS_COMPAT_OLD_NUTTX is disabled
+* CONFIG_SPIFFS_LEADING_SLASH=y
+
+### mkspiffs
+
+* Specify CONFIG_SPIFFS_NAME_MAX + 1 for SPIFFS_OBJ_NAME_LEN.
+* Specify 0 for SPIFFS_OBJ_META_LEN.
+
+### ESP-IDF spiffsgen.py
+
+* Specify CONFIG_SPIFFS_NAME_MAX + 1 for the --obj-name-len option.
+* Specify 0 for the --meta-len option.